In the context of the present application, a multi-phase stream comprises at least a co-existing vapour phase and a liquid phase, and optionally also a co-existing solid phase.
Such multi-phase streams may be produced from hydrocarbon wells, such as natural gas wells, in the form of a multi-phase hydrocarbon stream. The multi-phase hydrocarbon stream may comprise various components, including a variety of hydrocarbons, water, CO2, sulphides such as H2S and other elements or compounds.
Conventionally, multi-phase hydrocarbon streams may be carried over large distances from one or more hydrocarbon wells in a hydrocarbon reservoir to the apparatus which receives and processes the multi-phase streams. This can occur because, for instance the hydrocarbon wells are located off-shore and a pipeline is necessary to transport the multi-phase hydrocarbon stream to an on-shore processing facility.
Producing wells, either in the same hydrocarbon reservoir or from a different hydrocarbon reservoir, may provide multi-phase flows of significantly different characteristics in terms of compositions and properties, such as temperature and pressure. If such multi-phase flows have to be transported over a large distance before component separation can be carried out, economic limitations may require that such multi-phase flows of differing composition are carried in the same pipeline in a combined flow. Component separation must then be carried out on the combined flow. The separation facility will have one or more identical separation trains running in parallel to treat the combined flow.
Where large distances are involved between the well(s) and the separation facility, different multi-phase streams from different sets of hydrocarbon wells, which may be in the same or different hydrocarbon reservoirs, may be carried together in the same pipeline in order to reduce costs to render the hydrocarbon extraction economically viable. The use of a single long distance pipeline requires that the same method or methods used to ensure adequate flow of the multi-phase stream in the pipeline must be applied to all of the different multi-phase streams carried in the pipeline, if any steps are taken at all. These methods are known in the art as “flow assurance methods”. For instance, a pipeline can be insulated, heated or have hydrate inhibitor added to the multi-phase streams which it carries to minimise hydrate formation during transfer to the processing facility. The Ormen Lange field in the Norwegian Sea utilises such a flow assurance system, as described in the Journal of Petroleum Technology, August 2007, pages 51-61, in which a hydrate inhibitor is added to the multi-phase stream.
In addition, some hydrocarbon reservoirs can provide multi-phase hydrocarbon streams from different wells at different pressures. In such instances the pressure of the higher pressure multi-phase stream is normally reduced so that it can be added to the lower pressure multi-phase stream and transported along a single pipeline. This normally necessitates the re-pressurisation of at least the gaseous component of the multi-phase stream at the processing facility utilising a depletion compressor.